The present invention relates to an apparatus for controlling the amount of secondary air fed into an intake passage or into an exhaust passage of an internal combustion engine for controlling an equivalent air-fuel ratio (if an air-fuel passage from the intake passage through exhaust passage located upstream of an air-fuel ratio sensor is defined as a working fluid passage, the equivalent air-fuel ratio is defined as a ratio of the amount of air fed into the working fluid passage to the amount of fuel fed into the working fluid passage) within a predetermined range.
In the field of this art, a method is known in which the equivalent air-fuel ratio is detected by an air-fuel ratio sensor, for example, an oxygen concentration sensor for detecting the concentration of oxygen in the exhaust gas, and; then, secondary air is fed into an intake passage or into an exhaust passage of an internal combustion engine according to the detected equivalent air-fuel ratio, for maintaining the equivalent air-fuel ratio within a predetermined range which is near the stoichiometric air-fuel ratio, whereby the effect of purifying pollutants in a three-way catalytic converter disposed in the exhaust system is improved.
In a conventional apparatus for carrying out the above-mentioned method, the amount of secondary air to be injected into the engine is controlled by an air flow control valve disposed in a passage between an air pump and a secondary air injection mechanism. The air flow control valve is driven by an absolute pressure signal applied thereto through an electromagnetic valve which is adapted for switching the transmission of the absolute pressure on or off in response to an electrical signal provided from the air-fuel ratio sensor. More specifically, when a lean signal, which indicates that the equivalent air-fuel ratio is on the lean side of the stoichiometric air-fuel ratio, is provided from the air-fuel ratio sensor, a diaphragm of the air flow control valve is not actuated by the absolute pressure signal and is pressed by a return spring, so as to form a passage for discharging the air fed from the air pump into the atmosphere. Further, when a rich signal, which indicates that the equivalent air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio, is provided from the air-fuel ratio sensor, the diaphragm of the air flow control valve is actuated by the absolute pressure signal against the pressing force of the return spring, so as to form a passage for providing the air fed from the air pump to the secondary air injection mechanism. The absolute pressure signal may be a negative pressure signal using vacuum in an intake manifold of the engine as a carrier of the signal, or a positive pressure signal using, for example, the discharge pressure of the air pump as a carrier of the signal.
However, in the conventional apparatus of the above described type, when the duration of the lean signal exceeds a certain period, a time delay of the valve switching operation in response to the rich signal occurs. More specifically, when the duration of the lean signal exceeds a certain period, since the diaphragm of the air flow control valve completely returns to its initial position by receiving the pressing force caused by the return spring, the air flow control valve cannot immediately actuated to switch the passage of the secondary air in response to the next rich signal. Therefore, it is very difficult to control the equivalent air-fuel ratio within the predetermined range in this case.